This invention relates to a lateral semiconductor element in thin-film SOI technology, with an insulating later which rests on a substrate and is buried underneath a thin silicon film, on top of which the source, or anode, contact and the drain, or cathode, contact are mounted.
Semiconductor elements of the type described above have been known of for a long time, and are used, above all, in low voltage applications, e.g. as CMOS circuits. As a general rule, most circuits were contrived for demanding operating conditions (high temperatures, high particle radiation or electromagnetic radiation). Now, however, this technology is also increasingly finding its way into microprocessors and driver circuits.
Furthermore, these components may be either transistors or diodes. In this case, either an anode and a cathode contact or a source, a drain and a gate contact are mounted on the silicon film. As the position of the anode coincides functionally with the source contact of a transistor, and, accordingly, the cathode coincides with the drain contact, these terms will be used below in synonymous fashion, without thereby implying that the invention is restricted to a specific component.
At high operating voltages, the problem is that the blocking capacity of the buried insulator is insufficient, and the so-called back gate effect occurs, which means that the substrate has a control effect from below on the channel region of the SOI transistor and its threshold voltage is displaced. This means that with prior art semiconductor elements, it is virtually impossible to produce a plurality of components with a high blocking capacity using thin-film SOI technology.
These negative effects can be minimised or cancelled by means of very thick oxide layers. This approach is not satisfactory, however, because, for voltage of the order of 600 V, it is impossible to produce the oxides with a thin silicon film at reasonable technological or financial cost. By way of an alternative, a buried p-n junction can be created in the substrate, which protects the entire circuit from any harmful voltages which may occur from the underside of the switch. A p-n junction of this type, which can be produced e.g. by implantation through the silicon film and the buried oxide, is also referred to as a shield. It extends underneath and across the entire structure on a fixed potential.
The shield, however, only protects the circuit from voltages acting on the circuit from outside. The blocking voltages made possible by this are still relatively low, because the voltage breakdown is determined by the silicon film and the buried insulator.
The task of this invention is therefore to create a lateral semiconductor element of the type described above, which allows high blocking voltages inside the component, but is technologically simple to produce and economically viable to manufacture.
According to the invention, this task is solved in that the source or anode contact and the drain or cathode contact of the component each lie over separate shield areas within the substrate, with the anode contact being electrically connected to the substrate.
Like the prior art lateral semiconductor components in thin-film SOI technology, in this case the contacts are also disposed on a thin silicon film underneath which the insulating layer is buried. According to the invention, however, the anode, or source, gate and the gate contact of a transistor and the cathode, or drain, contact lie on two different shield areas floating with respect to each other. This not only reduces the space-charge region in the thin silicon film and the buried oxide, but in the substrate as well.
By means of the electrical contact between the anode contact and the substrate, the space-charge region is transferred to the substrate and thus homogenised in the silicon film.
The advantage of this semiconductor component lies in the high blocking capacity of its structure. The back gate effect no longer occurs because the shield region is structured and connected with the anode of each individual component. This means that back gate voltage can no longer occur.
The blocking capacity of this component is almost exclusively determined by the deep p-n junctions, i.e. shield rings, which, with an appropriate edge structure, make virtually any blocking voltages possible. The latch-up risk associated with a purely p-n insulation is eliminated.
The invention also offers the advantage that a plurality of high voltage switches can be integrated on a chip without any need for special epitaxy layers, oxide thicknesses of the buried insulator, or specific silicon film thicknesses. This means that bridgable systems can be built up on a chip and practically any vertical transistor structure integrated. Each transistor may contain a dielectrically insulated logic part, so that it is possible to manufacture whole half-bridge and full-bridge drivers with a control element.
To further increase the voltage sustaining capability, the two shield regions of the anode and cathode contacts are preferably insulated from each other by a buried edge structure disposed in-between them.
In one preferred embodiment, a floating field ring structure is used. With such floating field rings, which are also referred to as field limiting rings (FLR), one is dealing with an edge structure principle in which a blocking p-n junction is protected by upstream field rings, i.e. trough-shaped p-n junctions. These rings are not terminated and adjust themselves to a potential between maximum and minimum voltage.
In further preferred embodiments, the buried edge structure can also be contrived as a VLE or a JTE structure. In the case of the VLE structure (variation of lateral doping), a blocking p-n junction is extended by a pn junction. The doping which this requires declines in the lateral direction, which gives rise to a structure with an increasingly lower penetration depth and doping. In the case of a JTE structure (junction termination extension), the p-n junction, which joins up in the lateral direction, has a deeper penetration depth than the blocking p-n junction. In certain circumstances, however, the simple p-n junction disposed between the cathode and the anode contact is sufficient.
The electrical connection of the anode contact with the substrate is preferably produced directly by a metallic contact. In another preferred embodiment, this connection is produced by means of an electric component.
In addition to the anode contact, the cathode contact or, in the case of a transistor, the drain contact, is preferably also electrically connected with the substrate. It is essential that the shield region over which the main part of the blocking voltage declines is connected. If the blocking p-n junction is sufficiently connected, and if the SOI component adjusts to the horizontal voltage gradient, it may be possible to dispense with further connection of the SOI component.
If, for example, the substrate is n-doped and the shield is p-doped, the arrangement may only be operated with positive cathode-anode voltages, with the highest potential having to lie on a substrate contact which is attached at the back, or in another appropriate manner. In this case, the cathode shield forms a channel stop which closes off the structure in the substrate and prevents any expansion of the space-charge beyond the limits of the structure. In this case the cathode shield must be of the same doping type as the substrate, but oppositely doped with respect to the anode shield. If, for example, the substrate is n-doped and the anode shield is p-doped, the channel stop must be n+ doped.
The semiconductor component according to the invention is preferably a transistor, with the source and gate contacts on the one hand, and the drain contact on the other hand, lying over separate shield regions within the substrate.
In further preferred embodiments, the semiconductor component is a diode, a voltage-proof resistor or an IGBT, i.e. an insulated gate bipolar transistor.
In one semiconductor circuit with a transistor as the semiconductor component according to the invention, one is dealing with a level-shift transistor which is integrated in a driver circuit with a suitable vertical power component such as an IGBT, a DMOS or similar.
If the semiconductor component according to the invention is a diode, this can preferably be an SOI diode which, as a decoupling diode, e.g. for measuring current, is integrated in an evaluation circuit with a suitable vertical power component such as an IGBT, a DMOS or similar.
A plurality of transistors and diodes are preferably connected as semiconductor components according to the invention to form a bridge-driver circuit.